Concrete-filled Tubular Columns Research Articles

Ultimate resistance behavior of rectangular concrete-filled tubular beam-columns made of high-strength steel

High-strength steel is permitted in steel structures, whereas, further studies are needed to allow the use of high-strength steel in concrete-filled tubular (CFT) structures. In this study, twelve rectangular CFT columns using Q460-grade steel were tested under eccentric load and the test results were discussed. A nonlinear finite element model (FEM) was developed to predict the load-displacement and ultimate resistance behaviors of the test columns. Parametric studies were conducted using the verified FEM to investigate the influence of aspect ratios, steel strength, and width-to-thickness (h/t) ratio of steel plate. The ultimate resistance calculated by European Code (EC4), American Code (AISC 360) and Chinese Code (CECS 159) is compared with the FEM predictions to evaluate their feasibility in the use of higher aspect ratios, high-strength steel, and various h/t ratios. The results indicate that the three design codes are safe in the design of columns with higher aspect ratios. EC4 is conservative in the design of rectangular CFT columns with high-strength steel up to 690MPa under its h/t ratio limitation. AISC 360 method can accurately predict the maximum resistance of rectangular CFT columns with steel strength 550MPa. CECS 159 method is highly conservative and can be safely extended to the use of steel strength up to 690MPa and plate slenderness ratio 85235/fy.

Eccentric Compression Strength of Rectangular Concrete-Filled Tubular Columns Using High-Strength Steel Thin Plates

AbstractAn experimental study was performed to investigate the structural behavior of thin-walled rectangular concrete-filled tubular (RCFT) columns. This study mainly focused on the effects of a h...

Behavior of rectangular concrete-filled high-strength steel tubular columns with different aspect ratio

High-strength steel members are susceptible to local buckling. Rectangular concrete-filled tubular (CFT) columns could make full use of high-strength steel by restraining its buckling inward. This paper reports an experimental investigation of rectangular CFT columns using high-strength steel. Three current design codes are evaluated with the experiments both from this study and the previous literature. It indicates that EC4, AISC 360 and GB50936 tend to be unsafe incorporating high-strength steel and high aspect (h/b) ratio. Design recommendations of the width-to-thickness (h/t) ratio are proposed to account for the different h/b ratio effects for rectangular CFT columns.

Experimental behavior and design method of rectangular concrete-filled tubular columns using Q460 high-strength steel

This paper experimentally and theoretically investigates the behavior of rectangular concrete-filled tubular (CFT) columns using Q460-grade steel. Analysis of concrete contribution ratio (CCR), ductility and strength index (SI) was performed based on the experimental results. Finite element analysis was conducted to predict the behaviors of the test columns and to choose the suitable steel material model for Q460-grade steel. A comprehensive database including this study was established to evaluate the current design approaches for rectangular CFT columns using high steel strength (HSS). Augmentation factor is proposed to evaluate the effect of both the width-to-thickness ratio and the HSS. A new design method was developed and verified to extend the steel strength limitation for designing rectangular CFT columns to 460MPa in DB 29-57 based on this research.

Reflections on Mechanisms Affecting the Behavior of FRP-Concrete-Steel Double-Skin Tubular Columns

This paper presents the results of 20 hollow and concrete-filled double-skin tubular columns (DSTCs), which were tested as part of a comprehensive experimental program that was undertaken at The University of Adelaide on FRP-concrete steel DSTCs. The paper is aimed at providing important insights into the influence of two key parameters, namely the diameter of inner steel tube and presence/absence of a concrete-filling inside the inner steel tube, which play major roles in the column behavior through their influences on a series of interacting mechanisms that govern the complex system behavior. A detailed examination of the results yielded a number of important insights into the mechanisms that influence the compressive behavior of DSTCs.

Effects of axial and rotational restraints on concrete-filled tubular columns under fire

This paper presents a study of the effects of axial and rotational restraints on the fire response of concrete filled tubular (CFT) columns. The fiber model presented by the authors in a previous paper Ibañez et al. (2013) is employed to simulate the fire behavior of CFT columns within non-sway frames. Parametric studies are performed to analyze the influence of the different factors affecting the problem. Consequently, the opposite effects that the restraining frame has on the fire response of CFT columns are corroborated. On one hand, the restrained thermal elongation induces restraining forces which negatively affects the column. However, a beneficial effect is produced by the rotational restraint which positively modifies the boundary conditions of the heated column. Besides, another favorable effect comes from the gradual redistribution of the internal forces as the heated column loses its mechanical capacity, resulting in higher FRR. In a second step, current provisions given by Eurocode 4 Part 1.2 (EC4) are analyzed together with those given in the UK National Annex to Eurocode 4. Finally, these values are also used in the assessment of the simple calculation model presented by the authors in a previous paper Ibañez et al. (2015) and subsequently a proposal is made obtaining reasonably accurate but slightly safe results since the guidelines of CEN-Horizontal Group Fire were followed. As a result, the fire effective length value of 0.5L given by EC4 is found to lead to unsafe results, being more appropriate to adopt the value of 0.7L suggested in the UK National Annex.

Cyclic behavior of stiffened joints between concrete-filled steel tubular column and steel beam with narrow outer diaphragm and partial joint penetration welds

This paper presented an investigation on a stiffened joint in practical engineering which was between concrete-filled steel tubular column and steel beam with narrow outer diaphragm and partial joint penetration welds. Through the low-frequency cyclic loading test, the cyclic behavior and failure mode of the specimen were investigated. The results of the test indicated the failure mode and bearing capacity of the specimen which were influenced by the axial compression ratio of the concrete-filled tubular column. On the contrary, the inner diaphragm and inner stiffeners had limited impacts on the hysteretic behavior of the joint. There was no hysteresis damage fracture on the narrow outer diaphragm connected to the concrete-filled steel tubular column with partial joint penetration welds. Due to the excellent ductility and energy dissipating capacity, the proposed joint could be applied to the seismic design of high-rise buildings in highly intensive seismic region, but axial compression ratio should be controlled to avoid unfavorable failure modes.

Analytical evaluation of a modular CFT bridge pier according to directivity

This paper focuses on the analytical behavior of modular circular concrete-filled tubular (CFT) column with enhanced bracing details. To design a full-scale bridge pier of multiple circular concrete-filled tubes, numerical analysis was used to evaluate structural performance according to load directivity. In previous research (Ma <i>et al</i>. 2012, Shim <i>et al</i>. 2014), low cycle fatigue failure at bracing joints was observed, so enhanced bracing details to prevent premature failure are proposed in this analysis. The main purpose of this research is to investigate seismic performance for the diagonal direction load without premature failure at the joints when the structure reaches the ultimate load. The ABAQUS finite-element software is used to evaluate experimental performance. A quasi-static loading condition on a modular bridge pier is introduced to investigate structural performance. The results obtained from the analysis are evaluated by comparing with load.displacement responses from experiments. The concretefilled tubes with enhanced bracing details showed higher energy dissipation capacity and proper performance without connection failure for a diagonal load.

Test of extended thick-walled through-diaphragm connection to thick-walled CFT column

The strength and stiffness of the steel beams to concrete-filled tubular columns connections are significantly reduced if the thick-walled components are used. However, the thick-walled tubes used for columns can largely reduce the demand for space and increase the strength-to-weight ratio. This paper describes the cyclic performance of extended through-diaphragm connections between steel beams and thick-walled concrete-filled tubular columns improved with fillets around the diaphragm corners. Test on one full-scale connection was conducted to assess the seismic behavior of the connection in terms of strength, stiffness, ductility, deformation, energy dissipation, and strain distribution. It is shown that the fillets and extended through-diaphragm can alleviate the stress concentration in the connection and thus improve the seismic performance. The test results demonstrate that the through-diaphragm connections with thick-walled concrete-filled tubular columns can offer sufficient energy dissipation capacity and ductility appropriate for its potential application in seismic design.

Advanced materials for concrete-filled tubular columns and connections

The advantages of concrete-filled steel tubular (CFST) columns are well known at room and elevated temperatures, however, beyond a certain slenderness their load-bearing capacity starts to decrease. Besides, blind-bolts represent a proper system to allow endplate bolted connections to hollow steel tubular columns and CFST, although the resistance of the bolt shank conditions affects the performance of the connection. In this paper, the use of innovative materials is proposed as a method of enhancing of the load-bearing capacity for both CFST columns and connections. In this line, a first approach of the benefits using high strength steel, fire-resistant steel and geopolymer concrete applied for CFST columns in the fire situation is developed, obtaining better fire results although depending on the columns cross-sections configuration and the part where the advanced material is applied. Related to blind-bolts connections under fire conditions, the use of fire-resistant bolts is assessed. Their higher strength retention in fire could avoid the use of protection, but only in limited cases. Furthermore, a preliminary study on shape memory alloys in the blind-bolts is performed at room temperature and supporting cyclic pull-out loading.

Temperatures and thermal boundary conditions in reverse channel connections to concrete filled steel sections during standard and natural fire tests

The paper presents fire test and numerical simulation results of temperature distributions in reverse channel connections to concrete-filled tubular columns during standard and natural fire tests. The experiments included a furnace fire test with the composite frame subjected to increasing fire temperature according to the ISO 834 standard time–temperature curve and two natural fire tests in a full-scale structure including a cooling phase. These fire tests examined the effects of fire protection, different connection locations and different connection dimensions. Temperatures at different connection components were recorded. The recorded connection component temperatures were simulated by using the SAFIR fire engineering software to calibrate the thermal boundary conditions in different connection components. From the numerical simulation results, it has been concluded that radiation to the inner surfaces of the reverse channel and the adjacent part of the column tube is only from the gas volume bounded by these surfaces. A lower flame emissivity value than the standard value should be used. For simplicity, a value of εf=0.1 is proposed. Also a lower value of the convective heat transfer coefficient of 10W/m2K can be used in the connection area.

Three-Dimensional Cyclic Performance on New Ring-Beam Connection between Concrete-Filled Tubular Column and Reinforced-Concrete Beams

This paper introduces a new type of connection between concrete-filled steel tubular (CFST) column and reinforced concrete (RC) beam through a transitional RC ring beam encircling the CFST column. Four scaled specimens, representing an interior-column joint, were tested under bi-directional cyclic loads to examine the seismic performance and the influence of the reinforcement on the joint. Load-deformation response, failure mode, strain distribution, and energy dissipative performance were presented. Test results indicated ductile performance and the multi-directional nature of the force system in the ring beam. Finite element models were also calibrated against test results and results from parametric analysis confirmed the effect of tube diameter and longitudinal reinforcement in the ring beam on the structural performance.

Resistance and Flexure Behavior of Slender Welded Built-up Square CFT Column Using Internal Reinforced Steel Tube under Eccentric Loads

So far, square concrete filled tubular(CFT) columns have been used in a limited width thickness ratio. The reason is that local buckling occurs in steel tube easily. Once the local buckling occurs, the confinement effect of steel tube on concrete disappears. In this study, we developed welded built-up square steel tube with reinforcement which are placed at the center of the tube width acts as an anchor. 3 specimens of slender welded built-up square CFT columns and 3 specimens of slender welded built-up square steel tube columns were manufactured with parameters of width(B) of steel tube, width thickness ratio(B/t). we conducted a experimental test on the 6 specimens under eccentric load, and evaluated the structural resistance and behavior of 6 specimens.

Slender double-tube ultra-high strength concrete-filled tubular columns under ambient temperature and fire

This paper presents the results of an experimental campaign where both the room temperature and the fire resistance of six double-tube concrete filled steel tubular slender columns with different combinations of concrete strength are studied. Firstly, the ultimate axial load of the specimens at room temperature was experimentally obtained and afterwards the fire resistance of such columns subjected to a 20% of their load bearing capacity was measured. Given the reduced number of experimental results found in the literature on slender concrete filled tubular columns with double steel tubular cross-sections, the main objective of this paper is to compare the behaviour of such innovative cross-sections under ambient and high temperatures. The influence of filling the inner ring with concrete on the fire performance of these columns is studied in this paper, as well as the variation of thicknesses of the outer and inner steel tubes. Despite the fact that the tested columns are not covered by the scope of Eurocode 4, the current simple calculation models were applied in this paper in order to assess the validity of the standard to this typology of columns, unsafe results being found.

Circular Concrete-Filled Tubular Columns: State of the Art Oriented to the Vulnerability Assessment

The vulnerability of framed structures has been analyzed until recently from two different perspectives: Structural and socio-economical. For the sake of assessing the former, indexes and objective measurements have been proposed in the literature. These indexes include relatively accurate assessments of the strength, ductility, energy absorption, fire, blast response and resilience of the elements in order to define a higher-level structural magnitude. Similar approaches are performed with the latter when it comes to assessing damage, economical aspects, social and other important factors. On the other hand, concrete-filled tubes (CFT) have proven structurally efficient due to their relatively high strength-toweight ratio. Considerably complete state-of-the-art reviews are available for these members when it comes to analyzing their strength and overall or local buckling in static and/or dynamic responses. Reviews concerning important issues related to the structural vulnerability of those members are, however, scarce. In this paper, a state-of-the art dealing with the behavior of concrete-filled tubes is presented. The novelty of such approach is to present research concerning CFT but, in this case, from a structural vulnerability perspective (not socioeconomical), that is to say, summarizing references concerning seismic response, fire resistance, impact response and other main characteristics that are further used when defining the aforementioned indexes. Relevant numerical, experimental and theoretical studies presented in recent years are pinpointed as well as potential research trends.

Experimental investigation on the fire behaviour of rectangular and elliptical slender concrete-filled tubular columns

While the fire behaviour of concrete-filled steel tubular (CFST) columns with circular and square cross-section has been well established based on experimental programs and numerical investigations, the information about the fire behaviour of CFST columns with rectangular or elliptical cross-section is very scarce. Therefore, further research is needed in order to establish the structural behaviour of concrete-filled elliptical and rectangular hollow sections at elevated temperatures as a basis for the future development of new design guidance. In this paper, a series of slender CFST columns of rectangular and elliptical cross-section are tested at elevated temperatures under both concentric and eccentric loads, reaching large eccentricities. The effect of the load eccentricity and percentage of reinforcement is studied, considering both major and minor axis buckling. The influence of the cross-section shape, load eccentricity and percentage of reinforcement on the fire behaviour of these columns is investigated. The experimental results are subsequently used to assess the current design rules in Eurocode 4 Part 1.2 for these new section shapes.

Circular and square slender concrete-filled tubular columns under large eccentricities and fire

Previous investigations have revealed the unsafety of the current calculation methods in Eurocode 4 for evaluating the fire resistance of concrete-filled steel tubular (CFST) columns, which has given place to a movement in Europe for correcting the existing methods and developing new design rules. In order to support the development of new guidance, further experimental research is needed, especially concerning slender columns and large eccentricities. In this paper, the results of a series of fire tests on slender CFST columns of different section shape (circular and square) subjected to large eccentricities are presented. The influence of the cross-section shape, load eccentricity and percentage of reinforcement on the response of these columns at elevated temperatures is studied in this paper, focusing on the effect of large eccentricities combined with high slenderness. On the basis of the experimental results, the current design rules in Eurocode 4 are assessed, and a previous calculation method developed by the authors is also evaluated.

Fire resistance of double-skinned composite tubular columns including concrete confinement

Double-skinned composite tubular (DSCT) columns consisting of concrete cast between outer and inner tubes have been developed to overcome certain limitations of other columns, such as the large self-weight of concrete-filled tubular columns and lack of concrete confinement of hollow concrete-filled tubular columns. The strength and ductility of the column are enhanced by the continuous confining stress provided by the inner tube. Their excellent structural performances make them particularly suitable for applications in high-rise buildings. However, if a high-rise building is damaged by fire, the economic costs associated with building repair can be high. It is very important to put the fire-damaged building back into service with the minimum post-fire repair. Thus, to predict the status of a structure under fire, its behavior should be evaluated based on the fire duration. Studies on the fire resistance of DSCT columns have been carried out for this purpose. However, they have involved the performance of the entire system without considering the effects of the DSCT column’s components on the fire resistance. In this paper, the behavior of a DSCT column is investigated under an ISO-834 standard fire using an analytical method. The evaluation method for the fire resistance of a DSCT column utilizes a thermal analysis and Eurocode. In addition, the relationship between the DSCT column’s components and the fire resistance is investigated, considering the confining effect. Moreover, the behavior of the DSCT column is evaluated through parametric studies of the hollow ratio, thickness of the outer tube, and thickness of the inner tube.

Axial Load Test on Rectangular CFT Columns using High-Strength Steel and Slender Section

Abstract - An experimental study was performed for thin-walled recta ngular concrete-filled tubular (CFT) columns. The present study mainly focused on evaluation of the axial load-carrying capacity of concrete-filled tubular columns using high-strength steel and slender section. The test parameters were width-to-thickness ratio, concrete strength, steel yield strength, and the use of stiffeners. Five specimens were tested under monotonic axial loading. Although elastic local buckling occurred in the slender- section specimens with high-strength steel, the specimens exhibited considerable post-buckling reserve. The test results also satisfied the predictions of a current design code. The specimens strengthened with vertical stiffeners exhibited improved strength and ductility when compared with the un-stiffened specimens. Keywords - Rectangular CFT column, High-strength steel, Slender section, Concentric axial loading, StiffenerNote.-Discussion open until October 31, 2015. This manuscript for this paper was submitted for review and possible publication on May 18, 2014; revised October 25, 2014; approved on February 5, 2015.Copyright ⓒ 2015 by Korean Society of Steel Construction

Test and analysis on the seismic performance of a steel truss-to-circular CFT column sub-assembly

This paper presents an experimental and theoretical investigation on connection between steel truss and circular concrete-filled tubular (CFT) columns used for modern low-rise railway station platforms. Two full-scale cyclic tests were performed on connection sub-assembly and results showed that relative strength between the steel truss and column zone had a major effect on the subassembly's seismic performance and failure patterns. Results also indicated that a better deformability and energy dissipation capacity can be obtained if the column zone is slightly weaker than the steel truss. In addition, a simplified theoretical model was proposed to predict the experimental load–deformation histories and failure patterns. Satisfactory comparisons were observed.